Sandwich cathode lithium battery with high energy density

ABSTRACT

A lithium electrochemical cell with increased energy density is described. The electrochemical cell comprises an improved sandwich cathode design with a second cathode active material of a relatively high energy density but of a relatively low rate capability sandwiched between two current collectors and with a first cathode active material having a relatively low energy density but of a relatively high rate capability in contact with the opposite sides of the two current collectors. In addition, a cathode fabrication process is described that increases manufacturing efficiency. The cathode fabrication process comprises a process in which first and second cathode active materials are directly applied to opposite surfaces of a perforated current collector and laminated together. The present cathode design is useful for powering an implantable medical device requiring a high rate discharge application.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.15/290,595, filed on Oct. 11, 2016, now U.S. Pat. No. 10,263,240, whichclaims priority from U.S. Provisional Patent Application Serial No.62/239,867, filed Oct. 10, 2015.

TECHNICAL FIELD

This invention relates to the conversion of chemical energy toelectrical energy. In particular, the present invention relates to a newsandwich cathode design having a second cathode active material of arelatively high energy density but of a relatively low rate capabilitysandwiched between two current collectors, and with a first cathodeactive material having a relatively low energy density but of arelatively high rate capability in contact with the opposite sides ofthe current collectors. The present cathode design is useful forpowering an implantable medical device requiring a high rate dischargeapplication.

BACKGROUND OF THE INVENTION

Electrochemical cells provide electrical energy that powers a host ofelectronic devices such as external and implantable medical devices.Among these many medical devices powered by electrochemical cells areexternal medical drills and implantable cardiac defibrillators. Suchmedical devices generally require the delivery of a significant amountof current in a relatively short duration of time. Thus, these devicestypically require the use of electrochemical cells that comprise anincreased delivery capacity and an increased rate of charge delivery. Asdefined herein, “delivery capacity” is the maximum amount of electricalcurrent that can be drawn from a cell under a specific set ofconditions. The terms, “rate of charge delivery” and “rate capability”are defined herein as the maximum continuous or pulsed output current abattery can provide per unit of time. Thus, an increased rate of chargedelivery occurs when a cell discharges an increased amount of currentper unit of time.

Cathode chemistries such as carbon monofluoride (CF_(x)) have beendeveloped to provide increased discharge capacities that meet the powerdemands of external and implantable medical devices. CF_(x) cathodematerial is generally known to have a discharge capacity of about 875mAh/g, which is well suited for powering implantable medical devicesover long periods of time. However, electrochemical cells constructedwith cathodes comprised of carbon monofluoride are generally consideredto exhibit a relatively “low” rate capability. For example,electrochemical cells constructed with lithium anodes and CF_(x)cathodes typically exhibit rate capabilities from about 0.5 mA/cm² toabout 3 mA/cm². As such, electrochemical cells constructed withLi/CF_(x) couples are generally well suited for powering electricaldevices, like an implantable cardiac pacemaker that require power overlong periods of time at a relatively low discharge rate.

In contrast, electrochemical cells constructed with lithium anodes andcathodes comprising silver vanadium oxide (SVO) are generally consideredto exhibit a relatively “high” rate capability. Lithium cellsconstructed with SVO cathodes, in contrast to CF_(x) cathodes, generallyexhibit rate capabilities that range from about 25 mA/cm² to about 35mA/cm². As such, lithium electrochemical cells constructed with cathodescomprised of SVO are generally well suited to power devices that requirean increased rate capability, such as an implantable cardiacdefibrillator. However, lithium cells constructed with cathodescomprising SVO typically have a lower discharge capacity as compared tothose having cathodes comprising CF_(x). Silver vanadium oxide cathodematerial is generally known to have a discharge capacity of about 315mAh/g, which is significantly less than the discharge capacity of 875mAh/g for CF_(x) as previously discussed. Therefore, what is desired isan electrochemical cell having an electrode design that comprises both arelatively “high” discharge capacity material and a relatively “high”rate capability material that is capable of providing increaseddischarge capacity at a relatively high rate.

Prior art electrochemical cells comprising a lithium anode and a cathodeconstructed with both CF_(x) and SVO materials are disclosed in U.S.Pat. No. 6,551,747 to Gan, which is assigned to the assignee of thepresent application and incorporated herein by reference. These cellsare well suited for powering implantable medical devices, such asimplantable defibrillators, that require a relatively high chargecapacity with an increased discharge rate. The present inventionprovides a lithium electrochemical cell comprising a sandwich electrodedesign that incorporates both relatively high discharge capacity andrelatively high rate capability materials similar to that described bythe Gan '747 patent but having an increased energy density and improvedrate capability in comparison to prior art cells.

In addition, the present invention provides for an efficient assemblyprocess that is more conducive for manufacturing. Prior artelectrochemical cells, such as those disclosed in the Gan '747 patent,are assembled using a number of time consuming manual process steps. Theassembly process of the present invention provides for a more efficientprocess that eliminates many of the time consuming manual steps of theprior art assembly process, thereby reducing manufacturing time andexpense.

SUMMARY OF THE INVENTION

As is well known by those skilled in the art, an implantable cardiacdefibrillator is a device that requires a power source for a generallymedium rate, constant resistance load component provided by circuitsperforming such functions as, for example, the heart sensing and pacingfunctions. From time-to-time, the cardiac defibrillator may require agenerally high rate, pulse discharge load component that occurs, forexample, during charging of a capacitor in the defibrillator for thepurpose of delivering an electrical shock to the heart to treattachyarrhythmia, the irregular, rapid heartbeats that can be fatal ifleft uncorrected.

Accordingly, the object of the present invention is to improve theperformance of lithium electrochemical cells by providing an improvedelectrode design. Further objects of this invention include providing acell design for improving energy density by increasing electricalcapacity and improving rate capability.

To fulfill these needs, an improved sandwich cathode design having anincreased energy density that exhibits increased discharge capacitywithin a lithium electrochemical cell is provided. The cathode design ofthe present invention comprises a first cathode active material of arelatively high energy density but of a relatively low rate capability,for example CF_(x), sandwiched between two current collectors and with asecond cathode active material having a relatively low energy densitybut of a relatively high rate capability, for example SVO, in contactwith the opposite sides of the current collectors. Such an exemplarycathode design may comprise SVO/current collector/CF_(x)/currentcollector/SVO.

In addition to the improved cathode design, an improved method ofmanufacture is provided. Unlike prior art electrode manufacturingprocesses, the assembly process of the present invention eliminates manyinefficient manual manufacturing process steps, thereby decreasingmanufacturing time and cost.

These and other objects of the present invention will becomeincreasingly more apparent to those skilled in the art by reference tothe following description and to the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an embodiment of the cathode assembly process of thepresent invention.

FIG. 2 shows an embodiment of the anode assembly.

FIG. 3 is an explode view of an embodiment of an electrode assembly ofthe present invention.

FIG. 4 illustrates a perspective view of an embodiment of an assembledelectrode assembly shown in FIG. 3.

FIG. 5 is a graph that shows discharge capacity vs voltage pulse testingresults for test lithium electrochemical cells comprising a cathodeconstructed according to the present invention in comparison to controllithium electrochemical cells comprising a cathode constructed accordingto the prior art.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As used herein, the term “pulse” means a short burst of electricalcurrent of a significantly greater amplitude than that of a pre-pulsecurrent immediately prior to the pulse. A pulse train consists of atleast two pulses of electrical current delivered in relatively shortsuccession with or without open circuit rest between the pulses. Anexemplary pulse train may consist of four 10 second pulses (40 mA/cm²)with a 15 second rest between each pulse.

An electrochemical cell that possesses sufficient energy density anddischarge capacity required to meet the vigorous requirements ofimplantable medical devices comprises an anode of a metal selected fromGroups IA, IIA and IIIB of the Periodic Table of the Elements. Suchanode active materials include lithium, sodium, potassium, etc., andtheir alloys and intermetallic compounds including, for example, Li—Si,Li—Al, Li—B and Li—Si—B alloys and intermetallic compounds. Thepreferred anode comprises lithium. An alternate anode comprises alithium alloy such as a lithium-aluminum alloy.

The form of the anode may vary, but preferably the anode is a thin metalsheet or foil of the anode metal, pressed or rolled on a metallic anodecurrent collector, i.e., preferably comprising titanium, titanium alloyor nickel, to form an anode component. Copper, tungsten and tantalum arealso suitable materials for the anode current collector. In theexemplary cell of the present invention, the anode component has anextended tab or lead of the same material as the anode currentcollector, i.e., preferably nickel or titanium, integrally formedtherewith and contacted by a weld to a cell case of conductive metal ina case-negative electrical configuration. Alternatively, the anode maybe formed in some other geometry, such as a bobbin shape, cylinder orpellet to allow an alternate low surface cell design.

The electrochemical cell of the present invention further comprises acathode of electrically conductive material that serves as the otherelectrode of the cell. The cathode is preferably of solid materials andthe electrochemical reaction at the cathode involves conversion of ionsthat migrate from the anode to the cathode into atomic or molecularforms. The solid cathode may comprise a first active material of a metalelement, a metal oxide and a mixed metal oxide, and combinationsthereof, and a second active material of a carbonaceous chemistry. Themetal oxide and the mixed metal oxide of the first active material has arelatively lower energy density but a relatively higher rate capabilitythan the second active material.

The first active material is formed by the chemical addition, reaction,or otherwise intimate contact of various metal oxides and/or metalelements, preferably during thermal treatment, sol-gel formation,chemical vapor deposition or hydrothermal synthesis in mixed states. Theactive materials thereby produced contain metals and oxides of GroupsIB, IIB, IIIB, IVB, VB, VIB, VIIB and VIII, which includes the noblemetals and/or other oxide and sulfide compounds. A preferred cathodeactive material is a reaction product of at least silver and vanadium.

One preferred mixed metal oxide is a transition metal oxide having thegeneral formula SM_(x)V₂O_(y) where SM is a metal selected from GroupsIB to VIIB and VIII of the Periodic Table of Elements, wherein x isabout 0.30 to 2.0 and y is about 4.5 to 6.0 in the general formula. Byway of illustration, and in no way intended to be limiting, oneexemplary, cathode active material comprises silver vanadium oxidehaving the general formula Ag_(x)V₂O_(y) in any one of its many phases,i.e., β-phase silver vanadium oxide having in the general formula x=0.35and y=5.8, γ-phase silver vanadium oxide having in the general formulax=0.80 and y=5.40 and ε-phase silver vanadium oxide having in thegeneral formula x=1.0 and y=5.5, and combination and mixtures of phasesthereof. For a more detailed description of such cathode activematerials reference is made to U.S. Pat. No. 4,310,609 to Liang et al.,which is assigned to the assignee of the present invention andincorporated herein by reference. Another preferred composite transitionmetal oxide cathode material includes V₂O_(z) wherein z≤5 combined withAg₂O with silver in either the silver(II), silver(I) or silver(0)oxidation state.

It is further contemplated that the first active material of the presentsandwich cathode design is any material which has a relatively lowerenergy density but a relatively higher rate capability than the secondactive material. In addition to silver vanadium oxide and copper silvervanadium oxide, V₂O₅, MnO₂, LiCoO₂, LiNiO₂, LiMn₂O₄, TiS₂, Cu₂S, FeS,FeS₂, copper oxide, copper vanadium oxide, and mixtures thereof areuseful as the first active material.

The sandwich cathode design of the present invention further includes asecond active material of a relatively high energy density and arelatively low rate capability in comparison to the first cathode activematerial. The second active material is preferably a carbonaceouscompound prepared from carbon and fluorine, which includes graphitic andnon-graphitic forms of carbon, such as coke, charcoal or activatedcarbon. Fluorinated carbon is represented by the formula (CF_(x))_(n)wherein x varies between about 0.1 to 1.9 and preferably between about0.5 and 1.2, and (C₂F)_(n) wherein the n refers to the number of monomerunits which can vary widely. In addition to fluorinated carbon, Ag₂O,Ag₂O₂, CuF₂, Ag₂CrO₄, MnO₂ and even SVO itself are useful as the secondactive material.

Before fabrication into a sandwich electrode for incorporation into anelectrochemical cell according to the present invention, the first andsecond cathode active materials, prepared as described above, arepreferably mixed with a binder material and a solvent to createrespective first and second cathode material slurries. Binders such as,but not limited to a powdered fluoro-polymer, more preferably powderedpolytetrafluoroethylene or powdered polyvinylidene fluoride andsolvents, such as but not limited to, trimethylphosphate (TMP),dimethylformamide (DMF), dimethylacetamide (DMAc), tetramethylurea(TMU), dimethylsulfoxide (DMSO), or n-methyl-2-pyrrolidone (NMP) may bemixed with the respective first and second cathode active materials,i.e., SVO and CF_(x), to formulate the first and second cathodeslurries. In addition, up to about 10 weight percent of a conductivediluent may be added to the cathode slurries to improve conductivity.Suitable materials for this purpose include acetylene black, carbonblack and/or graphite or a metallic powder such as powdered nickel,aluminum, titanium and stainless steel.

In an embodiment, the first slurry may comprise a binder ofpolyvinylidene fluoride (PVDF), a solvent of n-methyl-2-pyrrolidone(NMP), carbon black and the SVO cathode active material. The secondslurry may comprise a binder of polyvinylidene fluoride, a solvent ofdimethylformamide (DMF), carbon black and the CF_(x) cathode activematerial. The applicants have discovered that proper selection of binderand solvent is beneficial in achieving adhesion of the respective firstand second cathode active materials to the opposed surfaces of thecathode current collector, particularly a cathode current collectorcomposed of aluminum.

FIG. 1 illustrates an overview of an embodiment of the cathode assemblyprocess of the present invention. In the embodiment, each of the firstand second cathode active materials, SVO and CF_(x), is applied directlyto an opposing surface of a perforated substrate 10 which serves as thecathode current collector. The substrate 10 is preferably composed of ametal selected from the group consisting of stainless steel, titanium,tantalum, platinum, gold, aluminum, cobalt nickel alloys,nickel-containing alloys, highly alloyed ferritic stainless steelcontaining molybdenum and chromium, and nickel-, chromium- andmolybdenum-containing alloys. The preferred substrate material for thecathode current collector is aluminum. In an embodiment, the perforatedsubstrate 10 may be in a continuous sheet form, such as a reel or roll.

As illustrated in FIG. 1, the substrate 10 that forms the cathodecurrent collector is preferably perforated in selected areas thereof. Ina preferred embodiment, the substrate 10 comprises a plurality ofperforations 12 that extend through the thickness of the substrate 10between opposing first and second substrate surfaces 14, 16. Other areas12A of the current collector 10 are preferably unperforated. In anembodiment, the perforations may be of a plurality of differentcross-sectional geometries. Examples of perforated shapes may include,but are not limited to, a circle, an oval, a rectangle, a star, or atriangle. In an embodiment, the perforation pattern may be formed by aphotoetch process or by mechanically pressing or punching the substrate10 prior to application of the cathode slurries. The plurality ofperforations 12 that extend through the thickness of the cathode currentcollector enable the exchange of ions between the first and secondcathode active materials, i.e., SVO and CF_(x), as both the first andsecond slurries comprising the SVO and CF_(x) cathode active materials,are preferably applied directly to the perforated surface of the currentcollector. In a preferred embodiment, the plurality of perforations 12create a cathode current collector having a percent open area thatranges from about 20 percent to about 30 percent, more preferably fromabout 21 percent to about 25 percent. As defined herein, the percentopen area is calculated within a 3 mm² region of the perforated area ofthe current collector. This preferred range of open area provides abalance between an open area that is large enough for the exchange ofions between the first and second cathode active materials whileensuring an open area that is small enough for adhesion of the first andsecond slurries to the surface of the cathode current collector.Additionally, the range of open area is tailored to provide for amechanically robust current collector.

In an embodiment, a first layer 18 of the first cathode slurrycomprising SVO is applied to the first surface 14 of the substrate 10 atthe perforations 12. A second layer 20 of the second cathode slurrycomprising CF_(x) is applied to the second, opposite surface 16 of theperforated current collector. Alternatively, the first and secondslurries may be simultaneously applied to the respective first andsecond perforated current collector surfaces 14, 16. In either case, thecathode assembly process of the present invention provides for a processin which the first and second slurries may be continuously applied to aroll or reel-to-reel of perforated cathode current collector.

In a preferred embodiment, the first and second slurries may be appliedto the current collector by coating, spreading, or screen printing therespective slurries directly to the perforated surface 14, 16 of thesubstrate 10. For example, a doctor blade may be used to apply a coatingof the slurry to the perforated current collector surface. In anembodiment, the first cathode slurry comprising SVO is applied to thefirst perforated surface 14 of the current collector at a weight basisthat ranges from about 10 mg/cm² to about 30 mg/cm², preferably fromabout 10 mg/cm² to about 20 mg/cm2, most preferably about 15 mg/cm², perlayer of the first cathode slurry. The second cathode slurry comprisingCF_(x) is applied to the second perforated surface 16 of the currentcollector at a weight basis that ranges from about 10 mg/cm² to about 40mg/cm², preferably from about 15 mg/cm² to about 30 mg/cm2, mostpreferably about 21 mg/cm², per layer of the second slurry.

In a preferred embodiment, as shown in FIG. 1, each of the first andsecond layers 18, 20 of the cathode active materials may compriserespective left and right cathode active material strips that areseparated by an intermediate gap 22 that is preferably unperforated andthat extends lengthwise along the substrate surfaces 14, 16. Inaddition, left and right margins 24, 26 that are preferably unperforatedseparate the first and second layers 18, 20 of the cathode activematerial from the side edges of the substrate 10. The intermediate gap22 and left and right margins 24, 26 are bare surfaces of the substrateof the cathode current collector on which the cathode active materialsare not present. In a preferred embodiment, the intermediate gap 22 mayhave a width that spans from about 1 cm to about 5 cm between the leftand right strips of cathode active materials 18, 20 on both sides 14, 16of the current collector. The left and right margins 24, 26 may have awidth that spans from about 1 cm to about 5 cm between the substrateside edge and the respective left and right strips of cathode activematerial.

In contrast, prior art cathode current collectors are fabricated fromindividual sheets of titanium on which the cathode active materials arepressed at pressures upwards of 30 tons per square inch onto aphotoetched perforated pattern. The prior art photoetching process is acumbersome time-consuming process that comprises masking and exposingindividual sheets of titanium to a chemical etch bath. Then, a carboncoating is applied to the surface of the titanium current collectorprior to the cathode active material being pressed thereon. This carboncoating process is used to inhibit corrosion of the titanium currentcollector by the formation of TiF from direct exposure to CF_(x). Incontrast, the assembly process of the present application comprisesdirect application of the cathode active slurries to the surface of acontinuous reel or roll of aluminum current collector that has beenpreviously perforated, either by a photoetching or a mechanical stampingprocess. The cathode assembly process therefore eliminates the carboncoating process of the prior art. In addition, the cathode assemblyprocess of the present invention eliminates the time-consuming processof masking and photoetching individual titanium sheets of the prior artprocess. Thus, the cathode assembly process of the present applicationprovides for a more efficient and cost-effective manufacturing process.

After the first and second active slurries are applied to theirrespective opposing first and second cathode current collector surfaces14, 16, the slurries are dried at a temperature that ranges from about100° C. to about 130° C. Alternatively, the first active slurry may bedried prior to the application of the second active slurry. Once dried,the first side of the cathode current collector preferably comprises afluoro-polymer binder at about 4 weight percent, a conductive diluent atabout 3 weight percent and about 93 weight percent of the first cathodeactive material, SVO. The second side of the current collectorpreferably comprises a fluoro-polymer binder at about 3 weight percent,a conductive diluent at about 3 weight percent and about 94 weightpercent of the second cathode active material, CF_(x).

Once the first and second active slurries are dry, the coated currentcollectors are cut into separate sheets 28, which are then stacked. Inan embodiment, two coated sheets 28 are stacked back to back so that thelayers of the CF_(x) material are positioned in contact with each other.Alternatively, one sheet 28 may be folded lengthwise along the gap 22 sothat the layers of CF_(x) material that reside opposite the gap 22, arefacing each other and are in physical contact. The folded or stackedsheets 28 are then welded together at least along one of the left andright margins 24, 26 to create a cathode sheet assembly 30.Subsequently, the welded cathode sheet assembly 30 is roll pressed in adirection perpendicular to the coating direction to laminate the sheetassembly 30 together. The preferred roll press direction helps preventcurling or warping of the assembly 30. In an embodiment, the roll pressis heated to a temperate that ranges between about 50° C. to about 100°C. In a preferred embodiment, the dried first slurry that comprises thefirst active material is pressed to a density that ranges from about 1.2g/cc to about 1.6 g/cc and the dried second slurry that comprises thesecond active material is pressed to a density that ranges from about2.8 g/cc to about 3.6 g/cc. Individual cathodes 32 are then removed fromthe assembly 30, such as by laser cutting or punching the cathodes in adesired shape. Such shapes may include, but are not limited to, arectangle, an oval or polygon that fits within the electrochemical cellcasing (not shown). The cathode 32 preferably comprises an integratedcathode tab 34 that is formed from the gap 22 of the metallic substrate.In an embodiment, the cathode is constructed to thereby comprise thefollowing sequence:SVO/current collector/CF_(x)/current collector/SVO

FIG. 2 illustrates an embodiment of an anode assembly 36 of the presentinvention. As shown, the anode 36 is constructed such that an anodecurrent collector 38, comprising a strip of metal, preferably nickel,having an upper edge 38A spaced from a lower edge 38B, both edgesextending to opposed end portions 38C, 38D is positioned in directcontact with the anode active material, such as lithium. As shown in theembodiment, the anode active material comprises a plurality of anodeleaves 40 having a scalloped shaped. However, it is contemplated thatthe form of the leaves 40 of anode active material may be of anunlimited shape. Examples of which may include, but are not limited to,a rectangle, an oval, or a multi-sided polygon shape. In a preferredembodiment, the leaves 40 of the anode active material are formed andshaped to fit within the casing of the electrochemical cell. Once theanode current collector 38 has been pressed to the anode activematerial, anode current collector tabs 42 are formed by bending the endsof the anode current collector 38. As illustrated in FIG. 3, the anodeassembly, including the current collector strips 38, is then folded inan accordion fashion between adjacent anode leaves 40. This allows forindividual cathodes 32 to be placed therebetween. As shown, individualcathodes 32 are positioned between folds 44 of the anode active material40 and current collector strips 38 to form an electrode assembly 46, anembodiment of which is illustrated in FIG. 4.

In order to prevent internal short circuit conditions, the sandwichcathode is separated from the Group IA, IIA or IIIB anode by a suitableseparator material. The separator is of electrically insulativematerial, and the separator material also is chemically unreactive withthe anode and cathode active materials and both chemically unreactivewith and insoluble in the electrolyte. In addition, the separatormaterial has a degree of porosity sufficient to allow flow there throughof the electrolyte during the electrochemical reaction of the cell.Illustrative separator material preferably includes polypropylene. Otherseparator materials may include fabrics woven from fluoropolymericfibers including polyvinylidine fluoride,polyethylenetetrafluoroethylene, and polyethylenechlorotrifluoroethyleneused either alone or laminated with a fluoropolymeric microporous film,non-woven glass, polypropylene, polyethylene, glass fiber materials,ceramics, polytetrafluoroethylene membrane commercially available underthe designation ZITEX (Chemplast Inc.), polypropylene membranecommercially available under the designation CELGARD (Celanese PlasticCompany, Inc.) and a membrane commercially available under thedesignation DEXIGLAS (C. H. Dexter, Div., Dexter Corp.).

The electrochemical cell of the present invention further includes anonaqueous, ionically conductive electrolyte which serves as a mediumfor migration of ions between the anode and the cathode electrodesduring the electrochemical reactions of the cell. The electrochemicalreaction at the electrodes involves conversion of ions in atomic ormolecular forms which migrate from the anode to the cathode. Thus,nonaqueous electrolytes suitable for the present invention aresubstantially inert to the anode and cathode materials, and they exhibitthose physical properties necessary for ionic transport, namely, lowviscosity, low surface tension and wettability.

A suitable electrolyte has an inorganic, ionically conductive saltdissolved in a nonaqueous solvent, and more preferably, the electrolyteincludes an ionizable alkali metal salt dissolved in a mixture ofaprotic organic solvents comprising a low viscosity solvent and a highpermittivity solvent. The inorganic, ionically conductive salt serves asthe vehicle for migration of the anode ions to intercalate or react withthe cathode active materials. Preferably, the ion forming alkali metalsalt is similar to the alkali metal comprising the anode.

In the case of an anode comprising lithium, the alkali metal salt of theelectrolyte is a lithium-based salt. Known lithium salts that are usefulas a vehicle for transport of alkali metal ions from the anode to thecathode include LiPF₆, LiBF₄, LiAsF₆, LiSbF₆, LiClO₄, LiO₂, LiAlCl₄,LiGaCl₄, LiC(SO₂ CF₃)₃, LiN(SO₂CF₃)₂, LiSCN, LiO₃SCF₃, LiC₆F₅SO₃,LiO₂CCF₃, LiSO₆F, LiB(C₆H₅)₄ and LiCF₃SO₃, and mixtures thereof.

Low viscosity solvents useful with the present invention include esters,linear and cyclic ethers and dialkyl carbonates such as tetrahydrofuran(THF), methyl acetate (MA), diglyme, trigylme, tetragylme, dimethylcarbonate (DMC), 1,2-dimethoxyethane (DME), 1,2-diethoxyethane (DEE),1-ethoxy,2-methoxyethane (EME), ethyl methyl carbonate, methyl propylcarbonate, ethyl propyl carbonate, diethyl carbonate, dipropylcarbonate, and mixtures thereof, and high permittivity solvents includecyclic carbonates, cyclic esters and cyclic amides such as propylenecarbonate (PC), ethylene carbonate (EC), butylene carbonate,acetonitrile, dimethyl sulfoxide, dimethyl formamide, dimethylacetamide, γ-valerolactone, γ-butyrolactone (GBL),N-methyl-pyrrolidinone (NMP), and mixtures thereof. In the presentinvention, the preferred anode is lithium metal and the preferredelectrolyte is 0.8M to 1.5M LiAsF₆ or LiPF₆ dissolved in a 30:70mixture, by volume, of propylene carbonate as the preferred highpermittivity solvent and 1,2-dimethoxyethane as the preferred lowviscosity solvent.

An important aspect of the present invention is that both the high ratecathode material (in this case the SVO material) and the high capacitymaterial, CF_(x) are positioned in direct contact with the currentcollector. The cathode current collector is preferably composed ofaluminum. This preferred current collector composition minimizes anypotential undesirable reaction that may occur between the cathode activematerial and the current collector. Prior art electrochemicalconstructions, such as those disclosed in U.S. Pat. No. 6,551,747, toGan et al., utilize a layer of carbon to inhibit possible corrosionbetween the CF_(x) and the surface of a titanium current collector. Thiscarbon layer is not ideal as it may impede ion transfer between the SVOand CF_(x) materials. In addition, the carbon occupies space which couldotherwise be used for active electrode materials.

Another embodiment of the present invention is that the cellconstruction has been optimized to remove electrochemically non-activematerial. For example, the thickness of the cathode current collector isreduced by about 57 percent to provide additional volume forelectrochemical active materials, such as CF_(x). For example, thethickness of the cathode current collector is reduced from about 0.07 mmto about 0.03 mm.

Another important aspect of the present invention is that a greaterratio of high capacity material having the low rate capability ispreferably positioned between two layers of high rate cathode material(either high or low capacities). Since the CF_(x) material hassignificantly higher volumetric capacity than that of SVO material,i.e., approximately 1.77 times greater, in order to optimize the finalcell capacity, the amount of CF_(x) material should be maximized and theamount of SVO material used in each electrode should be minimized to thepoint that the volume of SVO is still practical in engineering andacceptable in electrochemical performance. In an embodiment, thepreferred volume ratio of CF_(x) to SVO ranges from about 2 to about 12.The increased volume of CF_(x) significantly increases cell capacity.Thus, by optimizing the design of the cell, for example, by removingnon-active materials, such as decreasing the thickness of the currentcollector and removing the carbon from between the CF_(x) material andthe current collector surface, through the use of an aluminum currentcollector, additional volume within the cell is gained which is nowoccupied by active electrode materials.

The following examples describe the manner and process of anelectrochemical cell according to the present invention, and they setforth the best mode contemplated by the inventors of carrying out theinvention, but they are not to be construed as limiting.

EXAMPLE

Three lithium electrochemical test cells, each having a volume of about6.2 cc were constructed comprising the cathode assembly of the presentinvention. Each of the three test cells were constructed having acathode comprising a first cathode active layer having a 15.5 mg/cm²weight basis of a Ag₂V₄O₁₁ (SVO) mix containing 93% SVO, 4% of a PVDFbinder and 3% of a carbonaceous diluent, by weight, applied directly toa first surface of an aluminum cathode current collector. The other sideof the current collector had a 23.3 mg/cm² weight basis of a CF_(x) mixcontaining 94% active CF_(x), 3% of a PVDF binder and 3% of acarbonaceous diluent applied directly to the cathode current collectorsurface. The cathode was constructed having a total thickness of 0.49 mmwith each of the CF_(x) layers comprising 0.17 mm of the cathodethickness, each layer of SVO comprising 0.05 mm and each currentcollector comprising a thickness of 0.03 mm.

The test cells according to the present invention were assembled withthe SVO side of cathode facing a lithium anode and two layers ofpolypropylene separator disposed between the cathode and the anode. Eachcell was activated with an electrolyte of 1.29M LiAsF₆/PC:DME=30:70. Thetheoretical capacity of this cell was calculated to be 1914 mAh at a 2 Vbackground voltage cutoff.

The test cells according to the present invention were then pulsedischarged under a 3.23 A pulsing current (40 mA/cm²). The pulse trainsconsisted of four 10 second pulses with a 15 second rest between pulses.The pulse trains were applied to the cell every seven days. The pulsedischarge test results for the present invention test cells were thencompared to a prior art lithium electrochemical control cell modeled ata volume of 6.2 cc.

The prior art lithium electrochemical cell comprised a cathode having a28.1 mg/cm² weight basis of a Ag₂V₄O₁₁ (SVO) cathode mix consisting of94% active SVO, 3% of a PTFE binder and 3% of a carbonaceous diluent, byweight. The other side of the current collector had a 15.7 mg/cm² weightbasis of a CF_(x) mix containing 91% active CF_(x), 5% of a PTFE binderand 4% of a carbonaceous diluent. The cathode of the prior art controlcell was assembled per the prior art process of manually stacking an SVOblank in a press fixture, followed by a perforated carbon coatedtitanium current collector, then a CF_(x) blank followed by anotherperforated carbon coated titanium current collector and then finally asecond SVO blank. This stack assembly is subjected to a pressure ofabout 32 tons/in. The modeled cathode of the prior art cell had a totalthickness of about 0.54 mm with the CF_(x) blank comprising 0.22 mm ofthe total thickness and each SVO blank comprising 0.09 mm of thethickness and the current collector comprising about 0.14 mm of thetotal thickness. The prior art control cell was activated with anelectrolyte of 1.0M LiAsF₆/PC:DME=1:1. The theoretical capacity of thecell was calculated to be 1610 mAh at a 2.0V background voltage cutoff.

The prior art control cell was discharged in the same manner as the testcells of the present invention. The prior art control cell was pulsedischarged under a 3.23 A pulsing current (40 mA/cm²). The pulse trainsconsisted of four 10 second pulses with a 15 second rest between pulses.The pulse trains were applied to the control cell every seven days.

The test results are summarized in FIG. 5, which correlates celldischarge capacity (mAh) to cell voltage (mV) for the average of thethree test cells according to the present invention in comparison to theprior art control cell. As illustrated in the graph, the test cells, ingeneral, exhibited an average capacity greater than the modeled controlcell. As shown, the average discharge capacity at the pulse 4 minimumpotential of the test cells (curve 50) showed a greater averagedischarge capacity in comparison to the capacity of the pulse 4 minimumpotential of the modeled control cell (curve 48). For example, at a P1minimum voltage of about 1,500 mV, the modeled cell exhibited adischarge capacity of about 1,300 mAh.

In comparison, at a P1 minimum voltage of about 1,500 mV, the test cellsexhibited an average discharge capacity of about 1,750 mAh, which is anincrease of about 34.6 percent over the modeled control cell. Inaddition, at a pre-pulse voltage of about 2,500 mV, the modeled cellexhibited a discharge capacity of about 1,300 mAh (curve 52). Incomparison, at a pre-pulse voltage of about 2,500 mV, the test cellsexhibited an average capacity of about 1,500 mAh (curve 54), which is anincrease of about 15.4 percent over the modeled control cell.Furthermore, as illustrated in the graph of FIG. 5, the test cellsexhibited a greater end of life discharge capacity of about 2,000 mAh incomparison to the modeled end-of-life discharge capacity of about 1,500mAh for the same cell volume. The test cells delivered 0.65 Wh/cc (650Wh/L) to 2.55 V whereas the modeled cell delivered about 0.5 Wh/cc (500Wh/L) to 2.55V. Accordingly, this example clearly demonstrates theimprovement of using a SVO/CF_(x) sandwich cathode in a high rate,lithium electrochemical cell according to the present invention.

The above discussion discloses an improved sandwich electrode design ina lithium electrochemical cell capable of delivering increased dischargecapacity. With the sandwich design of the present invention, the highvolumetric capacity CF_(x) active material is quantitatively convertedinto or used as high-power energy of the SVO material. It is believedthat during high energy pulsing, all the discharge energy is provided bythe SVO material. Above the discharge voltage of the CF_(x) electrodematerial, only SVO electrode material is discharged with the SVOmaterial providing all of the discharge energy for pulsing as well asfor any background load discharging. Under these discharge conditions,the CF_(x) active material is polarized with respect to the SVO materialdischarge voltages. Then, when the lithium cells having the sandwichcathodes of the present invention are discharged to the working voltageof the CF_(x) material, both the SVO and CF_(x) active materials providethe energy for background load discharging. However, only the SVOmaterial provides energy for high rate pulse discharging. After the SVOactive material is pulse discharged, the potential of SVO material tendsto drop due to the loss of capacity. When the SVO background voltagedrops below the working voltage of the CF_(x) material, the SVO materialis believed to be charged by the CF_(x) material to bring the dischargevoltage of the sandwich cathode materials to an equal value. Therefore,it is believed that the SVO material acts as a rechargeable electrodewhile at the same time the CF_(x) material acts as a charger or energyreservoir. As a result, both active materials reach end of service lifeat the same time.

It is appreciated that various modifications to the inventive conceptsdescribed herein may be apparent to those of ordinary skill in the artwithout departing from the spirit and scope of the present invention asdefined by the appended claims.

What is claimed is:
 1. A cathode for an electrochemical cell, thecathode comprising: a) a first cathode current collector having opposedfirst and second major surfaces, and a second cathode current collectorhaving opposed third and fourth major surfaces, wherein a first tabportion and a second tab portion extend outwardly from the major facesof the respective first and second cathode current collectors; b) afirst electroactive material selected from the group of silver vanadiumoxide (SVO), copper silver vanadium oxide (CSVO), V₂O₅, MnO₂, LiCoO₂,LiNiO₂, LiMnO₂, TiS, FeS, FeS₂, and mixtures thereof, the firstelectroactive material contacting the first major surface of the firstcathode current collector and the fourth major surface of the secondcathode current collector; and c) a second electroactive materialdifferent than the first electroactive material and selected from thegroup of CF_(x), Ag₂O, Ag₂O₂, Ag₂CrO₄, MnO₂, SVO, and mixtures thereof,the second electroactive material contacting the second major surface ofthe first cathode current collector and the third major surface of thesecond cathode current collector, d) wherein the first and secondcathode current collectors are of the same material and are positionedso that the second electroactive material contacting the respectivesecond and third major surfaces contact each other with the firstelectroactive material contacting the respective first and second majorsurfaces facing outwardly.
 2. The electrochemical cell of claim 1,wherein the first and second tab portions are characterized as havingbeen formed by folding a sheet of electrically conductive material alonga fold line so that the first and second cathode current collectors areconnected to each other at least along the fold line.
 3. Theelectrochemical cell of claim 1, wherein the second electroactivematerials contacting the second and third major surfaces of therespective first and second cathode current collectors are characterizedas having been laminated to each other.
 4. The electrochemical cell ofclaim 1, wherein the first and second cathode current collectors areselected from the group consisting of stainless steel, titanium,tantalum, platinum, gold, aluminum, cobalt, molybdenum, nickel, andnickel alloys.
 5. The electrochemical cell of claim 1, wherein the firstand second cathode current collectors are perforated where the first,second, third and fourth electroactive materials contact them.
 6. Theelectrochemical cell of claim 5, wherein the first and second cathodecurrent collectors have a percent open area that ranges from about 20%to about 30% where the perforations reside.
 7. The electrochemical cellof claim 1, wherein the first and second tab portions of the respectivefirst and second cathode current collectors are unperforated.
 8. Theelectrochemical cell of claim 1, wherein the first electroactivematerial is a mixture comprising about 4 weight percent of afluoro-polymer binder, about 3 weight percent of a conductive material,and about 93 weight percent SVO, and wherein the second slurryelectroactive material is a mixture comprising about 3 weight percent ofa fluoro-polymer binder, about 3 weight percent of a conductivematerial, and about 94 weight percent CF_(x).
 9. The electrochemicalcell of claim 8, wherein the first and second fluoro-polymer bindersmixed with the first and second electroactive materials arepolytetrafluoroethylene or polyvinylidene fluoride.
 10. Theelectrochemical cell of claim 8, wherein the conductive material mixedwith the first electroactive material and with the second electroactivematerial is individually selected from the group consisting of acetyleneblack, carbon black, graphite, powdered nickel, powdered aluminum,powdered titanium, powdered stainless steel, and mixtures thereof. 11.The electrochemical cell of claim 1, wherein SVO as the firstelectroactive material has a density ranging from about 1.2 g/cc toabout 1.6 g/cc, and CF_(x) as the second electroactive material has adensity ranging from about 2.8 g/cc to about 3.6 g/cc.
 12. A cathode foran electrochemical cell, the cathode comprising: a) a first aluminumcathode current collector having opposed first and second majorsurfaces, and a second aluminum cathode current collector having opposedthird and fourth major surfaces, wherein a first tab portion and asecond tab portion extend outwardly from the major faces of therespective first and second cathode current collectors; b) silvervanadium oxide (SVO) contacting the first major surface of the firstcathode current collector and the fourth major surface of the secondcathode current collector; and c) fluorinated carbon (CF_(x)) contactingthe second major surface of the first cathode current collector and thethird major surface of the second cathode current collector, d) whereinthe first and second aluminum cathode current collectors are positionedso that the CF_(x) contacting the respective second and third majorsurfaces contact each other with the SVO contacting the respective firstand second major surfaces facing outwardly.
 13. The electrochemical cellof claim 12, wherein the first and second tab portions are characterizedas having been formed by folding a sheet of electrically conductivematerial along a fold line so that the first and second cathode currentcollectors are connected to each other at least along the fold line. 14.The electrochemical cell of claim 12, wherein the second electroactivematerials contacting the second and third major surfaces of therespective first and second aluminum cathode current collectors arecharacterized as having been laminated to each other.
 15. Theelectrochemical cell of claim 12, wherein the first and second aluminumcathode current collectors are perforated where the first, second, thirdand fourth electroactive materials contact them.
 16. The electrochemicalcell of claim 15, wherein the first and second cathode currentcollectors have a percent open area that ranges from about 20% to about30% where the perforations reside.
 17. The electrochemical cell of claim12, wherein the first and second tab portions of the respective firstand second cathode current collectors are unperforated.
 18. Theelectrochemical cell of claim 12, wherein the first electroactivematerial is a mixture comprising about 4 weight percent of afluoro-polymer binder, about 3 weight percent of a conductive material,and about 93 weight percent of the SVO, and wherein the second slurryelectroactive material is a mixture comprising about 3 weight percent ofa fluoro-polymer binder, about 3 weight percent of a conductivematerial, and about 94 weight percent of the CF_(x).
 19. Theelectrochemical cell of claim 18, wherein the first and secondfluoro-polymer binders mixed with the first and second electroactivematerials are polytetrafluoroethylene or polyvinylidene fluoride. 20.The electrochemical cell of claim 18, wherein the conductive materialmixed with the first electroactive material and with the secondelectroactive material is individually selected from the groupconsisting of acetylene black, carbon black, graphite, powdered nickel,powdered aluminum, powdered titanium, powdered stainless steel, andmixtures thereof.
 21. The electrochemical cell of claim 12, wherein theSVO has a density ranging from about 1.2 g/cc to about 1.6 g/cc, and theCF_(x) has a density ranging from about 2.8 g/cc to about 3.6 g/cc. 22.An electrochemical cell, comprising: a) a casing: b) an anode comprisinglithium; c) a cathode comprising: i) a first aluminum cathode currentcollector having opposed first and second major surfaces, and a secondaluminum cathode current collector having opposed third and fourth majorsurfaces, wherein a first tab portion and a second tab portion extendoutwardly from the major faces of the respective first and secondcathode current collectors; ii) silver vanadium oxide (SVO) contactingthe first major surface of the first cathode current collector and thefourth major surface of the second cathode current collector; and iii)fluorinated carbon (CFO contacting the second major surface of the firstcathode current collector and the third major surface of the secondcathode current collector, iv) wherein the first and second cathodealuminum current collectors are positioned so that the CF_(x) contactingthe respective second and third major surfaces contact each other withthe SVO contacting the respective first and second major surfaces facingoutwardly; and d) a separator disposed between the anode and the cathodeto prevent them from directly contacting each other; and e) anelectrolyte activating the anode and the cathode housed inside thecasing in electrochemical association with each other.
 23. Theelectrochemical cell of claim 22, wherein the first and second tabportions are characterized as having been formed by folding a sheet ofelectrically conductive material along a fold line so that the first andsecond cathode current collectors are connected to each other at leastalong the fold line.